3 research outputs found
Evaporation Kinetics of Laboratory-Generated Secondary Organic Aerosols at Elevated Relative Humidity
Secondary
organic aerosols (SOA) dominate atmospheric organic aerosols
that affect climate, air quality, and health. Recent studies indicate
that, contrary to previously held assumptions, at low relative humidity
(RH) these particles are semisolid and evaporate orders of magnitude
slower than expected. Elevated relative humidity has the potential
to affect significantly formation, properties, and atmospheric evolution
of SOA particles. Here we present a study of the effect of RH on the
room-temperature evaporation kinetics of SOA particles formed by ozonolysis
of α-pinene and limonene. Experiments were carried out on α-pinene
SOA particles generated, evaporated, and aged at <5%, 50 and 90%
RH, and on limonene SOA particles at <5% and 90% RH. We find that
in all cases evaporation begins with a relatively fast phase, during
which 30–70% of the particle mass evaporates in 2 h, followed
by a much slower evaporation rate. Evaporation kinetics at <5%
and 50% RH are nearly the same, while at 90% RH a slightly larger
fraction evaporates. In all cases, aging the particles prior to inducing
evaporation reduces the evaporative losses; with aging at elevated
RH leading to a more significant effect. In all cases, the observed
SOA evaporation is nearly size-independent
Modeling Novel Aqueous Particle and Cloud Chemistry Processes of Biomass Burning Phenols and Their Potential to Form Secondary Organic Aerosols
Phenols emitted from biomass burning contribute significantly
to
secondary organic aerosol (SOA) formation through the partitioning
of semivolatile products formed from gas-phase chemistry and multiphase
chemistry in aerosol liquid water and clouds. The aqueous-phase SOA
(aqSOA) formed via hydroxyl radical (•OH), singlet
molecular oxygen (1O2*), and triplet excited
states of organic compounds (3C*), which oxidize dissolved
phenols in the aqueous phase, might play a significant role in the
evolution of organic aerosol (OA). However, a quantitative and predictive
understanding of aqSOA has been challenging. Here, we develop a stand-alone
box model to investigate the formation of SOA from gas-phase •OH chemistry and aqSOA formed by the dissolution of
phenols followed by their aqueous-phase reactions with •OH, 1O2*, and 3C* in cloud droplets
and aerosol liquid water. We investigate four phenolic compounds,
i.e., phenol, guaiacol, syringol, and guaiacyl acetone (GA), which
represent some of the key potential sources of aqSOA from biomass
burning in clouds. For the same initial precursor organic gas that
dissolves in aerosol/cloud liquid water and subsequently reacts with
aqueous phase oxidants, we predict that the aqSOA formation potential
(defined as aqSOA formed per unit dissolved organic gas concentration)
of these phenols is higher than that of isoprene-epoxydiol (IEPOX),
a well-known aqSOA precursor. Cloud droplets can dissolve a broader
range of soluble phenols compared to aqueous aerosols, since the liquid
water contents of aerosols are orders of magnitude smaller than cloud
droplets. Our simulations suggest that highly soluble and reactive
multifunctional phenols like GA would predominantly undergo cloud
chemistry within cloud layers, while gas-phase chemistry is likely
to be more important for less soluble phenols. But in the absence
of clouds, the condensation of low-volatility products from gas-phase
oxidation followed by their reversible partitioning to organic aerosols
dominates SOA formation, while the SOA formed through aqueous aerosol
chemistry increases with relative humidity (RH), approaching 40% of
the sum of gas and aqueous aerosol chemistry at 95% RH for GA. Our
model developments of biomass-burning phenols and their aqueous chemistry
can be readily implemented in regional and global atmospheric chemistry
models to investigate the aqueous aerosol and cloud chemistry of biomass-burning
organic gases in the atmosphere
Parameterized Yields of Semivolatile Products from Isoprene Oxidation under Different NO<sub><i>x</i></sub> Levels: Impacts of Chemical Aging and Wall-Loss of Reactive Gases
We
developed a parametrizable box model to empirically derive the
yields of semivolatile products from VOC oxidation using chamber measurements,
while explicitly accounting for the multigenerational chemical aging
processes (such as the gas-phase fragmentation and functionalization
and aerosol-phase oligomerization and photolysis) under different
NO<sub><i>x</i></sub> levels and the loss of particles and
gases to chamber walls. Using the oxidation of isoprene as an example,
we showed that the assumptions regarding the NO<sub><i>x</i></sub>-sensitive, multigenerational aging processes of VOC oxidation
products have large impacts on the parametrized product yields and
SOA formation. We derived sets of semivolatile product yields from
isoprene oxidation under different NO<sub><i>x</i></sub> levels. However, we stress that these product yields must be used
in conjunction with the corresponding multigenerational aging schemes
in chemical transport models. As more mechanistic insights regarding
SOA formation from VOC oxidation emerge, our box model can be expanded
to include more explicit chemical aging processes and help ultimately
bridge the gap between the process-based understanding of SOA formation
from VOC oxidation and the bulk-yield parametrizations used in chemical
transport models